In recent years, environmental problems, such as global warming, have come to light. The importance of natural gas, which has high H/C ratio compared with other hydrocarbon fuels, coal, etc., can suppress the amount of carbon dioxide emissions, and also has rich reserves, has been reviewed. It is expected that the demand for natural gas will increase more and more in the future. Under such circumstances, there are great many small and middle gas fields found in the regions of Southeast Asia, Oceania and so forth, which however are still left undeveloped due to their locations of distant places having no infrastructure such as a pipeline and an LNG plant, requiring a huge amount of investment for the infrastructure being in comparable to their minable reserves, so that their developments have been desired to be processed. As one of the effective developing means of the gas fields, after natural gas is converted into syngas, development of a technology of converting natural gas into syngas and then the syngas into liquid hydrocarbon fuels, such as kerosene and light oil, which are excellent in transportability and handling ability, by using a Fischer-Tropsch (F-T) synthesis reaction is energetically performed in many places.
F-T synthesis reaction formula:nCO+2nH2→CH2n+nH2O
This F-T synthesis reaction is an exothermic reaction which converts a syngas into a hydrocarbon by using a catalyst, so it is very important to effectively remove heat of reaction for a stable operation of the plant. As effective reaction types up to now, there are gas-phase synthesis processes (a fixed-bed, an entrained-bed, a fluidized-bed reactor) and a liquid-phase synthesis process (a slurry-bed reactor). Although these processes have various features, attention has recently been paid to a slurry bed liquid-phase synthesis process which is high in heat removal efficiency, and does not cause accumulation of generated a high boiling point hydrocarbon onto a catalyst, or the following plugging of a reaction tube. This process is being developed energetically.
Generally, it is preferable that the activity of a catalyst be higher and higher. However, especially in the slurry-bed reactor, in order to maintain an excellent slurry fluidized state, there exists a limit in that it is necessary to set the concentration of slurry to a certain value or less. Therefore, making the activity of the catalyst higher becomes a very important factor in expanding the degree of freedom of process design. The activity of various F-T synthesis catalysts which have been reported up to now is at most about 1 (kg-hydrocarbon/kg-catalyst/hr) with a general index of productivity of a liquid hydrocarbon which carbon number is 5 or more. This is not enough from the above viewpoint (R. Oukaci et al: Applied Catalysis A: General, 186 (1999) 129 to 144).
As one of methods for improving the activity of the catalyst, there is a report that it is effective to reduce the content of sodium in silica used as a catalyst support (J. Chen: Cuihua Xuebao Vol. 21, 2000, Pages 169 to 171). However, in this report, there is only comparison between the catalyst which sodium content is below 0.01 mass % and the catalyst which sodium content is about 0.3 mass %, but there is no specific description that an effect is exhibited by reducing the sodium content to a certain extent.
Additionally, as a result of elaborate studies to an effect of impurities, such as alkali metals and alkaline earth metals, on the activity of a catalyst, there is an example in which the activity is greatly improved as compared with a conventional catalyst by adopting a catalyst which impurity concentration is within a certain range (refer to Japanese Unexamined Patent Application Publication No. 2004-322085).
Additionally, generally, the particle size of the catalyst for the F-T synthesis reaction is preferably smaller from a viewpoint of lowering the possibility that the diffusion of heat or substance becomes rate-determining level. However, in the F-T synthesis reaction by the slurry-bed reactor, a high boiling point hydrocarbon among hydrocarbons to be generated are accumulated within a reaction container. Therefore, the solid-liquid separating operation between the catalyst and a product is necessarily needed. Thus, when the particle size of the catalyst is too small, the problem that the efficiency of a separating operation is greatly degraded occurs. Hence, an optimal particle size range exists in the catalyst for the slurry-bed reactor, and is generally about 20 to 250 μm. As a mean particle size, 40 to 150 μm is preferable. However, as shown below, the catalyst may be broken or powdered during a reaction, and the particle size may become small. Thus, attention is required.
That is, in the F-T synthesis reaction in the slurry-bed reactor, operation is often made at a relatively high material-gas superficial velocity (0.1 m/second or more), and catalyst particles collide violently with each other during a reaction. Therefore, when physical strength or attrition resistance (powdering resistance) is not enough, the particle size of the catalyst may decrease during a reaction, and inconvenience may be caused in the above separating operation. Moreover, a lot of water is obtained as a by-product in the F-T synthesis reaction. However, when a catalyst which has low water resistance, and is apt to cause strength reduction, breakage, and powdering due to water is used, the particle size of the catalyst may become fine during a reaction. Thus, inconvenience will be caused in the separating operation similarly to the above.
Additionally, generally, in order to have the optimal particle size as described above, the catalyst for the slurry-bed reactor is pulverized to be adjusted in particle size, and then it is provided for practical use. Meanwhile, in such a crushed catalyst, often, precracking is performed and acute projections are created. Thus, the catalyst is inferior in mechanical strength or attrition resistance. Therefore, when being used for the slurry bed F-T synthesis reaction, the catalyst is broken, and is finely powdered. As a result, there is a drawback in that the separation between the high boiling point hydrocarbon to be generated and the catalyst became significantly difficult. Additionally, when porous silica is used as a catalyst support for the F-T synthesis reaction, it is widely known that a catalyst with relatively high activity is obtained. However, when adjustment of particle size by crushing has been performed, due to the reasons as described above, often, silica is low in water resistance and is apt to be broken and powdered by the existence of water as well as strength being degraded. Therefore, this often becomes a problem especially in the slurry-bed reactor.
Additionally, under a reaction atmosphere in which the water which is generated as a by-product by the F-T reaction exists in large quantities (especially under an atmosphere of high CO conversion), a phenomenon that catalytic activity decreases occurs and the decrease is because cobalt silicates are formed mainly at an interface between loaded cobalt that is an active metal and a silica support, or the loaded cobalt itself is oxidized or sintering occurs. This became a problem. Additionally, since this phenomenon also leads to accelerating the deterioration rate of the catalyst with the lapse of time, i.e., reducing catalyst life, this became a factor which increases operating cost. These can be described by the wording that the water resistance of cobalt particles showing activity is low. Especially under an atmosphere where the CO conversion is high, the partial pressure of by-product water increases, and thereby, deterioration rate increases. As a result, the above decrease in catalytic activity appears noticeably. However, even under an atmosphere where the CO conversion is not as high as 40 to 60%, progress of decrease in catalytic activity will be made at a relatively low speed according to the partial pressure of the by-product water. Accordingly, it is important to improve water resistance even on the condition that the CO conversion is relatively low from a viewpoint of the catalyst life. With respect to the inhibition of formation of cobalt silicate, and an improvement in activity, it is considered that addition of zirconium is effective. However, in order to exhibit the effect of the zirconium, a large amount zirconium which is about half of the mass of cobalt is required, or even if a large amount of zirconium is added, the effect thereof was not satisfactory (refer to U.S. Pat. No. 6,740,621 B2).
The factors involved in a decrease in catalytic activity may include precipitation of carbon on the surface of cobalt or at an interface between supported cobalt and the silica support in addition to the above. By covering the surface of cobalt with a carbon component, the surface area of cobalt which can contact a material-gas will be reduced, and the catalytic activity will decrease. In addition; poisoning by a sulfur component, a nitrogen component, etc. in the material-gas, or sintering whereby cobalt metal may agglomerate during a reaction is common.
When a catalyst which activity has decreased falls below an activity level due to the factors, it is necessary to replace or regenerate the catalyst in order to maintain the performance of a reaction process. In the slurry-bed reactor, there is a feature that the catalyst which activity has decreased can be replaced without stopping a reaction. However, if it is possible to regenerate the catalyst which activity has decreased, a replacement catalyst for maintaining reaction performance is not needed, or the amount of replacement can be reduced. Therefore, production cost can be reduced.